If I can give you a hint from the Contemporary side of the house.

I "slope" all my waistbands. That is because my kilts are designed to be worn at, or a couple of inches below the navel. If you look at pair of jeans from the side you will see that the back can be significatly higher than the front.

Try this.

Make the kilt about two inches longer than your drop measurments. When you get the fell tapers and aprons sewn down, wrap the kilt around your friend. Then take a nice wide belt and have her put on the belt so the top of it is at the point wgere the kilts waistband will be.

Check to see that the hem is horizontal with a yardstick all the way around.

Now take you tailors chalk and mark the kilt right at the top of the belt.

Have her remove the kilt and lay it out on your lay-out table. Duplicate the tapered chalk line from the outer apron to the inner apron.

The chalk line should look like a long sine wave, or double "S" curve.

That is where you attach the waistband. Cut the sloped waist just as you would for a straight one.

I have successfully made kilts with a 4 inch differance between the front and back.

I hope this helps.